Icelandic town evacuated due to concerns about volcanic eruption.

The town of Grindavík on the Reykjanes Peninsula, Iceland, which has about 3000 permanent residents plus a thriving tourism industry, was evacuated on Saturday 11 November 2023, following a sharp increase in Earthquakes in the area, which geologists fear may be linked to an imminent eruption from the nearby Fagradalsfjall Volcano. Iceland, which is an Earthquake-prone country with numerous active volcanic fields, began to suffer an increase in seismic activity on 25 October, since when it has suffered around 22 000 Earthquakes, with the Reykjanes Peninsula being particularly affected. In the 24 hours before the evacuation was ordered, 1400 Earthquakes were reported beneath Iceland, 880 of them beneath the Reykjanes Peninsula.

A crack in a road in the town of Grindavík on the Reykjanes Peninsula, Iceland, caused by an Earthquake this week. Ragnar Visage/RÚV/Reuters.

The evacuations were triggered by the discovery of a magma tunnel running directly beneath the town at a depth of about 1.5 km. This is about 12 km long, originating near Stóra-Skógfell hill, and running beneath the town and some way out to sea. There appears to be a considerable amount of lava in this tunnel, leading to fears that any eruption would be considerably larger than the Vestmannaeyjar eruption of 1973, in which a 3 km long fissure opened, resulting in the destruction of 400 homes, and an ashfall which covered most of Iceland, reaching 5 m deep in places, although only a single person lost their life. 

The length of the magma tunnel and its position partly under the town of Grindavík.  Kristrún Eyjólfsdóttir/RÚV.

Iceland lies directly upon the Mid-Atlantic Ridge, a chain of (mostly) submerged volcanoes running the length of the Atlantic Ocean along which the ocean is splitting apart, with new material forming at the fringes of the North American and European Plates beneath the sea (or, in Iceland, above it). The Atlantic is spreading at an average rate of 25 mm per year, with new seafloor being produced along the rift volcanically, i.e. by basaltic magma erupting from below. The ridge itself takes the form of a chain of volcanic mountains running the length of the ocean, fed by the upwelling of magma beneath the diverging plates. In places this produces volcanic activity above the waves, in the Azores, on Iceland and on Jan Mayen Island. All of this results in considerable Earth-movement beneath Iceland, where Earthquakes are a frequent event.

The passage of the Mid-Atlantic Ridge beneath Iceland. NOAA National Geophysical Data Center.

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Understanding how caldera-collapse drove the 2014-15 Bárdarbunga eruption in Iceland.

Between August 2014 and February 2015 the Bárdarbunga volcanic system in Iceland underwent the largest volcanic eruption in Iceland (or anywhere else in Europe) since the 1783–1784 eruption on Mount Laki, producing 1.4 cubic kilometers of basaltic lava, not from the main caldera, which is burried beneath the Vatnajökull Ice Cap, but rather from the Holuhruan Vent Field, which is 48 km from the caldera and is connected to it by a network of fissures. During the course of this eruption the ice sheet above the caldera subsided by 65 m, leading volcanologists to conclude that the eruption was driven by the collapse of the caldera into the underlying magma chamber, only the seventh such eruption observed sinc the advent of seismic monitoring in the early twentieth century; the earlier cadera collapse eruptions being Katmai 1912, Fernandina 1969, Tolbachik 1976, Pinatubo 1991, Miyakejima 2000, and La Reunion 2007, all of which were substantial volcanic episodes.

 Lava eruptiong from the Holuhruan Vent Field in Sepember 2014. Eggert Norddahl/Bergsveinn Norddahl/VolcanoCafe.

In a paper published in the journal Science on 15 July 2016, a team of scientists led by Magnús Gudmundsson of the Institute of Earth Sciences at the University of Iceland describe the results of a study of the 2014-15 Bárdarbunga eruption using data from seismic monitoring stations and aerial radar observations made of the caldera during the eruption.

The eruption began on 16 August 2014 with a series of small earthquakes beneath the southeastern part of the caldera, followed by the development of a new rift, which originally propagated to the southeast, reaching 7 km from the caldera within 15 hours. This rift then changed direction, migrating northeast and reachin the Holuhraun Vent Field in two weeks (the rift eventually spread to 41 km beyond the vent field). Material passing from the magma chamber through the rift reached the  Holuhraun Vent Field on 31 August 2016, leading to the onset of the visible eruptive episode.

The loss of material into the new rift system led the magma chamber to begin to deflate, placing stress on the rocks around the margin of the caldera, which were now forced to support the weight of rock and ice above the chamber. On 23 August 2016 the first of a series of small tremors on the northern part of the rim as recorded, followed by the spreading of such activity around the rim over the next few days. This in turn was followed by substantial subsidance in the central part of the caldera, as new faults developed around the rim, enablimg a plug of material above the magma chamber to subside into the chamber. This in turn forced further magma out of the chamber and into the rift system, driving further volcanism on the Holuhraun Vent Field.

 The Bárdarbunga caldera and the lateral magma flow path to the Holuhraun eruption site. (A) Aerial view of the ice-filled Bárdarbunga caldera on 24 October 2014, view from the north. (B) The effusive eruption in Holuhraun, about 40 km to the northeast of the caldera. (C) A schematic cross section through the caldera and along the lateral subterranean flow path between the magma reservoir and the surface. Gudmundsson et al. (2016).

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Glacial outburst flood from Grímsvötn's Ice Lake                                                              On 27 September 2015 volcanologists from the Icelandic Meteorological Office noticed a drop in the level of ice above a subglacial lake in the Grímsvötn caldera (a lake beneath the Vatnajökull ice-cap kept liquid by heat rising from the magma chamber...

Sulphur Dioxide emissions from the 2014-15 Holuhraun Lava Field Eruption.                      In mid-August 2014 seismic monitoring stations in Iceland began to record small Earth-tremors beneath the Bárðarbunga Volcano, which rises through the Vatnajökull Glacier...

Magnitude 5.4 Earthquake beneath the Vatnajökull Glacier in Iceland.                         The Icelandic Met Office, which also monitors seismic activity, recorded a Magnitude 5.4 Earthquake at a depth of 4.1 KM beneath the Vatnajökull Glacier...

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